Many polycyclic aromatic hydrocarbons (PAHs) are known to be toxic and carcinogenic for humans, and their contamination of soils and aquifers is of great environmental concern. Some microorganisms, mainly from the genera Pseudomonas and Mycobacterium, were found to be capable of transforming and degrading PAHs. These abilities may be useful in removal of PAHs in the environment. The successful application of bacteria to the bioremediation of PAHs contaminated sites requires a deeper understanding of how microbial PAHs degradation proceeds. In this review, the bacteria involved and the metabolic pathways for the degradation of many PAHs are summarized and the biological aspects of the PAHs bioremediation are discussed.